1. Field of the Invention
The present invention relates to methylamylketoxime (MAKO) blocked polyisocyanates, which have a significantly reduced viscosity when compared to corresponding polyisocyanates blocked with methylethylketoxime (MEKO), and to the use of these polyisocyanates in high solids coating compositions.
2. Description of the Prior Art
In view of the increasing number of regulations requiring a reduction in the amount of volatile organic solvents present in coating compositions, many attempts have been made to formulate high solids coating compositions. One possibility is to optimize the selection of solvents or blends of solvents to determine those which provide the greatest reduction in viscosity based on the weight of the solvent. Another possibility is to formulate the coating compositions with lower viscosity raw materials.
Coating compositions based on a blocked polyisocyanate component and a polyol component are known. The purpose of the blocking agent prevents the polyisocyanate component from reacting with the polyol component prematurely. An advantage of these coating compositions over those based on fully reacted polyurethanes is that the viscosity of the individual components is less than the viscosity of the fully reacted polyurethane. During subsequent curing of the coating composition at elevated temperatures the blocking agent is released and the polyisocyanate and polyol components react to form a high molecular weight polyurethane.
One of the preferred classes of blocking agents are the oximes. These blocking agents provide the good storage stability which is generally associated with caprolactam blocking agents, yet release the blocking agent at much lower temperatures than those needed for caprolactam blocking agents. By good storage stability it is meant that the blocking agents prevent a premature reaction from occurring between isocyanate groups and hydroxyl groups which would increase the viscosity and possibly gel the coating composition.
Even though coating compositions based on oxime-blocked polyisocyanates and a polyol component have a lower viscosity than fully reacted polyurethanes, it is an object of the present invention to further reduce the viscosity of these systems without affecting the properties of the coatings obtained therefrom.
These objectives may be achieved in accordance with the present invention by using methylamyl ketoxime (MAKO) as the blocking agent for the polyisocyanate component. Certain polyisocyanates blocked with MAKO possess lower viscosities when compared to the same polyisocyanates blocked with methylethyl ketoxime (MEKO).
U.S. Pat. Nos. 4,486,556 and 4,708,995 are representative of a series of patents relating to electrodeposition coatings. Both of these patents describe the preparation of blocked polyisocyanates by blocking a biuret group-containing polyisocyanate based on 1,6-hexamethylene diisocyanate and an isocyanurate group-containing polyisocyanate based on toluene diisocyanate with MAKO. U.S. Pat. No. 4,847,346 describes the preparation of an MAKO-blocked polyisocyanate adduct wherein the adduct is prepared by reacting toluene diisocyanate with trimethylol propane. U.S. Pat. No. 4,808,658 describes the preparation of a polyisocyanate adduct blocked with 2-octanone oxime wherein the adduct is prepared by reacting isophorone diisocyanate with trimethylol propane. U.S. Pat. No. 4,794,154 describes the preparation of an MAKO-blocked polyisocyanate adduct wherein the adduct is prepared by reacting .alpha., .alpha., .alpha.',.alpha.'-tetramethyl-xylylene diisocyanate with trimethylol propane.
U.S. Pat. No. 4,596,744 is directed to the use of oximes containing at least 7 carbon atoms, preferably MAKO, as blocking agents. The preferred polyisocyanates are adducts prepared by reacting toluene diisocyanate with trimethylol propane and isocyanurate group-containing polyisocyanates based on 1,6-hexamethylene diisocyanate. However, in view of the fact that neither of these polyisocyanate adducts was blocked with MAKO in the examples of this reference, there is no recognition of the improvement in viscosity which may be obtained when the specific polyisocyanates of the present invention are blocked with MAKO.